This is western feminism's dirty little secret. Behind the glorious image of the have-it-all woman in the Armani suit, with a Gucci briefcase on one arm and a baby tucked under the other, too often lies a tale of the oppression of another woman. Domestic servitude has only been escaped by passing it down to another cadre of oppressed women. Battalions of low-paid women - in America most of them foreign - have taken up the domestic duties, along with the dirty washing, discarded by professional women who have fled the home. Liberation for high-fliers breaking through glass ceilings is only possible because of a flotilla of unseen, unheard women who care for their children, clean their homes and cook their meals while they live liberated like men.

This is a book to tear at the heart and wrench with guilt many women who already feel they are juggling their lives on a knife-edge. Their own deep anxieties about their children and their high-pressured lives are all too often passed on to the women who work for them, making them exceptionally bad employers.

In America this is a story of the mass importation of a precious new raw material - care and love - from the third world. Take one typical case: Rowena Bautista left a village in the Philippines to work as a domestic in Washington DC - one of about 800,000 legal household workers (plus armies of illegals). In her basement room she has photos of four children, two of her own whom she has left behind and two of her American charges to whom she has to some extent transferred her love and care.

She left her own children in the care of their grandmother five years ago when the youngest, Clinton, was only three: she could find no work to provide for them. The children's grandmother is herself so hard-pressed that she works as a teacher from 7am to 9pm each day, so Rowena has hired a local woman to cook, clean and care for the family in her long absence. (In her turn, that woman leaves her own child in the care of a very elderly grandmother.) Rowena hasn't managed to get home to the Philippines for the last two Christmases, but the family relies on the money she sends.

Rowena calls the American child she tends "my baby". She says: "I give Noa what I can't give my own children." Last time she saw her own son, he turned away from her, asking resentfully: "Why did you come back?"

The distress and damage done to such abandoned children is well-documented in this collection of research. A series of essays edited by two of the great American writers on work, it exposes a deeply shocking underworld of globally exploited women. This is one of those moments when things that are known but unspoken are dragged out into the light of day. Chapter after chapter reveals how women's traditional roles, rejected by western women, are now being filled by wickedly treated other mothers. Their love is bought, they give everything to their charges and yet they are often sacked on a whim, never to see their child charges again.

Imported cleaners, cooks, old-age carers, nannies and housemaids are joined by mail-order brides for men who like the submissive "old-fashioned" values from the east. (That chapter is aptly called "Highly Educated Overseas Brides and Low-Wage US Husbands".) And there are the sex-workers and sex-slaves, some who knew what they were in for, others who were tricked or kidnapped. Horror stories abound, including child sex tourism.

Countries such as the Philippines have become economically dependent on the remittances women domestic workers send home. They may leave behind men whose skills are in less demand in the west: demoralised by unemployment, some husbands turn to drink and gambling, wasting all the hard-earned money their wives send, leaving the children worse off than if their mothers had stayed home.

This is a most brutal example of the force of globalisation, draining even love away from poor countries. It is the final depredation, exploiting the last resources the third world has left to sell - motherhood and sex.

Since this is an American book, I checked the official number of domestic workers in Britain: it is 154,000 and not rising, though a great many more certainly work in the black economy. There have been enough cases of diplomats bringing in visa-slaves as domestics to make it clear that many of the same abuses happen here. In the UK the social injustice is mainly indigenous: professional women pass their un-wanted domestic work on to poorer British women at pitiful rates of pay. Only the richest 20% of working women can afford to buy childcare, paying very low wages to minders or nursery assistants. Well-paid nannies are confined to the topmost echelons.

What is to be done? Barbara Ehrenreich's ground-breaking book Nickel and Dimed exposed the impossibility of living on the minimum wage in the US: one of her most memorable jobs was working for The Maids, a domestic cleaning service. Here, recalling that starvation drudgery, she offers a ferociously forensic dissection of everything wrong with a corrupted capitalism that has led to this exploitation of third-world women.

Hers is a devastating feminist critique, almost as savage about high-earning women who pass on their domestic duties as she is about the sexist world in which all domestic work is consigned to women in the first place. In the "chore wars" of 1970s feminism, she says, men won. They took on almost no extra housework or childcare. "Enter then the cleaning lady as dea ex machina, restoring tranquility as well as order to the home," she writes. Marriage-guidance counsellors now recommend them as an alternative to squabbling.

In the US, this is a race as well as a class issue: maids are mainly black, reinforcing rich kids' views that black means servant: a little white girl in a supermarket trolley passing a little black girl exclaims: "Oh, look Mommy, a baby maid!" The mistress-maid relationship is fraught, and Ehrenreich describes how an "overclass" has become deskilled in any domestic knowledge, unable to cook or clean, with children who would "suffocate in their own detritus" without someone to pick up after them. She twists the knife in overprivileged women who have "something better" to do with their time in a society where the rich get richer and the poor poorer.

In Britain, this debate revolves around the state's failure to provide universal childcare with well-paid nursery assistants: there's nothing wrong with equal women working for good pay as respected childcare professionals. In the US, state provision is not even worth talking about.

But Ehrenreich's eloquent moral fury is primarily directed at a capitalism that exploits every last drop of blood of the weak, wherever they are in the world, whatever they have to sell, even a mother's love. Unregulated, out-of-control capitalism creates a long-hours culture in which women cannot compete and still be mothers. Above all, the fault is with men who still refuse to take an equal share in everything domestic - thinking, planning and doing. If they did, the nature of work would change.

This deeply disturbing book reaches right to the dark heart of society's worst dysfunctions, with stories to make you weep with outrage. If postfeminism means that it's all right for some other woman to be exploited instead of you, this should fire up some of that good old-time passion. Feminism always was a revolutionary project, and Ehrenreich bemoans a project left uncompleted: "Sooner or later someone else will have to finish the job."

· Polly Toynbee's Hard Work: Life in Low-Pay Britain is published by Bloomsbury.